Fluorine-18 incorporation and radiometal coordination in macropa ligands for PET imaging and targeted alpha therapy

Abstract

The development of theranostic agents for radiopharmaceuticals based on therapeutic alpha emitters marks an important clinical need. We describe a strategy for the development of theranostic agents of this type via the functionalization of the ligand with the diagnostic radionuclide fluorine-18. An analogue of macropa, an 18-membered macrocyclic chelator with high affinity for alpha therapeutic radiometals, was synthesized and its complexation properties with metal ions were determined. The new macropa-F ligand was used for quantitative radiometal complexation with lead-203 and bismuth-207, as surrogates for their alpha-emitting radioisotopes. As a diagnostic partner, a radiofluorinated macropa ligand was used for quantitative bismuth(III) and lead(II) complexation. All fluorine-18 and radiometal complexes are highly stable in human serum over several days. This study presents a new proof-of-principle approach for developing theranostic agents based on alpha-emitting radionuclides and fluorine-18.

Figure 1:

¹H NMR spectra (400 MHz, 298 K, D₂O, pD = 7, MeCN as internal reference) of a) macropa-F, b) [Bi(macropa-F)]⁺, and c) [Pb(macropa-F)]. The diagnostic aliphatic resonances are labelled with asterisks for each compound and indicate the diastereotopic benzylic methylene linker protons.

Figure 2:

X-ray crystal structure of [Bi(macropa-F)]⁺. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms, counter ions, and solvent molecules are excluded for clarity. Grey = C, red = O, yellow = F, blue = N, purple = Bi.

Figure 3:

a) Normalized radio-HPLC traces of the independently injected radiofluorinated complexes [¹⁸F][Pb(macropa-F)] and [¹⁸F][Bi(macropa-F)]⁺. b) Analytical HPLC traces of the independently injected non-radioactive reference metal complexes [Pb(macropa-F)] and [Bi(macropa-F)]⁺ for comparison.

Figure 4:

Summary of the radiolabelling efficiency experiments and kinetic stability studies of macropa and macropa-F with ²⁰³Pb and ²⁰⁷Bi. Experiments were performed in triplicate. a)/b) Concentration-dependent radiolabelling studies with different concentrations of macropa and macropa-F and ²⁰³Pb or ²⁰⁷Bi. c) Human serum stability studies of [²⁰³Pb][Pb(macropa)] and [²⁰³Pb][Pb(macropa-F)] complexes at different time points. d) Human serum stability studies of [²⁰⁷Bi][Bi(macropa)]⁺ and [²⁰⁷Bi][Bi(macropa-F)]⁺ complexes at different time points.

Scheme 1:

a) Synthesis route of a macropa-derived precursor macropa-Me₂-NO₂ for fluorine-18 radiolabelling and b) synthesis of the corresponding non-radioactive reference 4 and macropa-F for radiometal complexation.